This invention relates to gas-insulated bushings, and more specifically relates to a novel gas-insulated bushing structure having a reduced diameter by virtue of an improved dielectric arrangement for the bus. The invention is specifically an improvement of the air entrance bushing shown in U.S. Pat. No. 3,934,071 in the name of Tahiliani, dated Jan. 20, 1976 and owned by the assignee of the present application.
Gas-insulated bushings as shown in the above patent are relatively simple in construction and consist essentially of a porcelain or other insulation material shell which surrounds a tubular conductor. The space between the conductor and the porcelain is filled with compressed gas, such as sulfur hexafluoride, which is at a gas pressure which is usually about the same as that used in the gas-insulated equipment which is connected to the bushing.
Capacitive grading is generally not used for this type of bushing and this results in a tendency for high dielectric stress to develop at the ground side of the bushing.
The bushing is more capable of supporting dielectric stress on its interior than on its exterior, since the interior of the bushing is filled with electronegative gas under pressure. Therefore, the dielectric stress appearing on the external surface of the bushing, whether it be porcelain or any suitable insulation plastic, tends to be the limiting factor in the performance of the bushing and the bushing is generally constructed in a manner to reduce the external surface stresses.
External surface stress has been limited to acceptable limits by using relatively large diameter insulation bodies. This is a satisfactory solution to the problem at lower voltages, where the insulator costs are relatively low, but at higher voltages the very large diameter porcelain or other insulation material becomes very expensive and difficult to manufacture. The porcelain diameter can be reduced by arrangements such as that disclosed in above-noted U.S. Pat. No. 3,934,071. However, the arrangement of the above-noted patent still requires a relatively large diameter insulation housing because of the relatively high external stress on the outer surface of the insulation body.